In the particle beam therapy, therapy is implemented by irradiating a charged particle beam onto a diseased site, which is a therapy subject, so as to kill tissues of the diseased site; in order to deliver a sufficient dose to the tissues of the diseased site without causing damage to the peripheral tissues thereof, there is required a particle beam therapy system that can appropriately control an irradiation dose and irradiation coverage (referred to as an irradiation field, hereinafter). In a so-called broad-irradiation-type particle beam therapy system, among particle beam therapy systems, that utilizes an irradiation nozzle provided with a scanning electromagnet such as a wobbler electromagnet, the irradiation nozzle enlarges the irradiation field and a multi-leaf collimator that changes a penetration shape is disposed in the enlarged irradiation field, so that an irradiation field corresponding to the shape of a diseased site is formed.
A multi-leaf collimator, in which two leaf lines formed of leaf plates laminated in the thickness direction are arranged in such a way as to face each other and the leaf plates are driven in a direction in which they approach each other or in a direction in which they are separated from each other, forms a predetermined penetration shape. Accordingly, by controlling the respective physical positions of the leaf plates, an irradiation field can readily be formed. However, in the case of a linear-driven leaf plate, in the contour portion that is away from the center of the irradiation field, a so-called penumbra is caused in which a charged particle beam having an angle toward the spreading direction hits part of the end face of the leaf plate and hence the dose of the charged particle beam is continuously attenuated. Thus, a so-called cone-shaped multi-leaf collimator has been proposed (e.g., refer to Patent Document 1 or 2) in which the spread of a beam is taken into consideration and leaves formed in a shape obtained through division at the side surface of an arc or a cone are driven on a circular orbit.